Shaft coupling

[Sachakins]

I suggest 2 sprockets & a toothed belt.

Better with a vee belt.
Problem with toothed sprockets if anything jams, they won't slip like a vee belt, causing more damage.

 

[ChaiLatte]

...with a coupler with say 1mm backlash, instead of the reverse direction happening at 20,15,10,5, it happens at 21,16,11 and 6...

Please explain where the 21 comes from.

 

[Phill05]

Please explain where the 21 comes from.

You press key 2 then press key 1 and Viola you have 21

 

[Dabop]

Please explain where the 21 comes from.

From the starting position of course....

Using the same example
Your stepper motor etc starts from a known position (home position), advances so many steps to travel the 5mm (say 5000 steps)- drills a hole say, moves to 10mmm, drills another etc etc all the way to 20mm from the starting position and moves to the far end of its traveling position (say 25mm away...)...
Now it moves over say 5mm (you are making a mini pegboard lol) and reverses, drilling another series of holes next to them, while traveling in the reverse direction...

But because there is 1mm backlash (slop) in the coupling, when it comes back- the first hole drilled (supposed to be at 20mm from the start position) is actually at 21mm from the start position (because it had to 'take up the slack' of 1mm before the drill head started to actually move- the motor 'thinks' it has moved back 5mm, but in reality has only moved 4mm and taken up that 1mm 'slack' in the coupling... and it repeats it all the way down that next row- all out by 1mm from where they are supposed to be...

This is why minimising backlash in CNC is so critical (unless you have the $$$ to get actual servos instead of steppers)

In the OP's case- backlash is totally unimportant of course, but strength is...